The invention relates to vented filling plugs for electric storage cells.
During the charging and discharging of electric cells a potentially explosive mixture of gases is frequently liberated. In the case of lead acid accumulators, a stoichiometric mixture of hydrogen and oxygen is liberated. An external spark, static discharge or other high temperature source can start a flame which passes down the inside of the cell and the resultant explosion can rupture the cell casing, spraying acid around and causing considerable damage and danger.
According to the present invention, a vented filling plug for an electric storage cell has a vent path including a flame trap through which gas may enter or leave the cell arranged so as, in use, to communicate with the interior of the cell at a point above the normal electrolyte level, and a filling path through which electrolyte may be poured into the cell arranged so as, in use, to communicate with the interior of the cell at a point below the normal electrolyte level.
Preferably the flame trap in the vent path includes a liquid trap. Alternatively, or in addition, the flame trap may include a foraminous partition, for example of ceramic material.
Preferably the filling path also includes a liquid trap. In a preferred construction, the liquid trap is positioned below the normal electrolyte level and constructed and arranged so that, if the electrolyte is allowed to fall below the level at which the filling path communicates with the interior of the cell, the liquid trap is left at least partially filled with electrolyte.
Thus, even if the user is negligent about keeping the cell topped up with electrolyte, the presence of two liquid traps will render it extremely unlikely that a flame outside the cell will be able to travel into the cell causing it to explode